Focus servo-circuit with ordered reference level setting means

ABSTRACT

In order to focus light beams which are projected onto an optical recording medium, the focusing optical system is moved in a vertical direction to the surface of the recording medium. A focus error signal produced by receiving the light returning from the recording medium is compared in turn with a predetermined plurality of reference levels. A focus servo-means is operated when this signal reaches the reference level within the focus pull-in region after passing through a predetermined order of reference levels.

This application is a Division of Ser. No. 832,810 filed Feb. 24, 1986,now U.S. Pat. No. 4,740,679.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

This invention relates to a focus servo-circuit for focusing an opticalsystem to optically record or reproduce signals in a photodisc orphotomagnetic disc device.

Photomagnetic effect and photodiffraction phenomenon are used by anoptical means which writes information into an optically reproduciblemedium (which shall be called a disc hereinafter) or which readsinformation out of an already recorded disc. The photomagnetic effect isapplied to a photomagnetic disc device so that the information may befreely written in and erased. The photodiffraction phenomenon is alreadyused by a compact disc player or photodisc player. A focus servo-meanswhich projects minutely throttled light beams onto a disc irrespectiveof the face deformation of the disc is indispensable to such a meansthat writes-in and reads-out or a means that only writes-in or onlyreads-out as is mentioned above. Generally, the focus servo-means isprovided to detect a focus error signal (which shall be mentioned as anFE signal hereinafter). The focus servo-means is to make a spot of thebeams follow the displacement of the deformation of the face of the discon the basis of the detected FE signal. Some of the methods used toobtain this signal are an astigmatism method, a skew beam method, aknife edge method and a critical angle method.

FIG. 1 shows an example of an FE signal characteristic obtained by theabove mentioned astigmatism method. In the diagram, the ordinaterepresents the levels of the FE signal S_(FE) and the abscissarepresents the relative distances between an objective lens and thedisc. The signal will be obtained when the disc moves through the entirefocus range (depth) of the objective lens. The signal will be adifference signal in the amount of light between paired opposite lightreceiving surfaces when a spot of beams is projected onto aphotodetector made by dividing a light receiving surface into quarters.The photodetector serves as a the detecting means. The one feature ofthis signal is that, for example, the signal will represent the maximumlevel point S₁ when the disc approaches the focus within the focaldistance of the objective lens. The signal will also represent theminimum level point S₂ when the disc approaches the focus from far away(an opposite case also exits). An exact or just focus point L_(J) islocated substantially in the intermediate position between the relativedistances L₁ and L₂ at which the respective maximum and minimum levelsare obtained. The signal level at L_(J) is a substantially intermediatelevel between S₁ and S₂. Therefore, if an actuator means, which drivesthe objective lens, is applied with an FE signal level between S₁ toS_(J) when the relative distance with the objective lens and disc isbetwee L₁ to L_(J) and is applied with an FE signal level between S_(J)to S₂ when the relative distance is betwee L_(J) to L₂, the objectivelens will always be held at the exact or just focus point L_(J) and thedistances L₁ to L₂ will represent a focus pull-in region.

The device of the prior example, is so difficult to initially preciselyset the disc in the pull-in region, with respect to the optical system,that the focus position is detected by moving the objective lens withrespect to the set disc to pass through the focus position by graduallyapproaching from a separate position from the disc. When the objectivelens is detected as having entered the object focus pull-in region, theservo-means will then be applied.

However, the focus servo-means pull-in region is so narrow that, even ifthe servo-loop is closed as soon as the focus position is detected, dueto the transmission delay of the circuit, when the servo-means isactually applied, the objective lens will have been already displacedout of the pull-in region. Even if the servo-means is applied in thisstate, the focus state will not be able to be set and therefore amis-operation occurs.

In the above mentioned prior example, usually the objective lens ismoved in a direction X approaching the disc from a position far from thedisc. However as shown in FIG. 1, the FE signal has a quasi-focus pointc having the same level as S_(J) and therefore a defective focalposition is detected.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a focus servo-circuitwhich can be positively set at a just or an exact focus point.

Another object of the present invention is to provide a focusservo-circuit which can be effectively set an exact focus point.

A further object of the present invention is to provide a focusservo-circuit having a wide range of applications.

According to the present invention, a means is provided for comparing aplurality of ordered reference levels which are set in response to thevarious level variation characteristics of the focus error signals whenan optical system, which projects light beams onto a recording medium,is moved in a vertical direction to the surface of the recording medium.The various levels of the focus error signals, when the optical systemis actually moved, is provided so that, when respective reference levelsare crossed in a predetermined order and the reference level within thefocus pull-in region is reached, the focus servo-means will be operated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a characteristic diagram showing focus error signalcharacteristics produced using an astigmatism method.

FIGS. 2 to 5 relate to the first embodiment of the present invention.

FIG. 2 is a circuit block diagram showing an example of a principleformation of the first embodiment.

FIG. 3 is a formation diagram showing a focus servo-circuit of the firstembodiment.

FIG. 4 is a flow chart showing the operation of the first embodiment.

FIGS. 5a and 5b are explanatory diagrams showing the operation of thefirst embodiment.

FIG. 6 is a characteristic diagram showing focus error signalcharacteristics according to the third embodiment of the presentinvention.

FIGS. 7a-d are flow charts for setting reference levels in the fourthembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First, the principal formation of the present invention shall beexplained with reference to FIG. 2.

The focus servo-mechanism is a means of servo-controlling an exact focusof an optical system A₁ having an objective lens. The servo-mechanismdrives the optical system in directions X (near/close) and Y (far/away)relative to a disc-shaped recording medium A₂. The servo-mechanism isprovided with an actuator A₃ which drives at least the objective lens ofthe optical system A₁ in the X and Y directions. An error signaldetecting means A₄ detects a focus error signal S_(FE) based onreflected beams from the recording medium A₂. A switching means A₅switches the output of the detecting means A₄ on and off. A drivingcircuit means A₆ amplifies the electric power of the focus error signalS_(FE) coming through the switching means 5 and feeds the signal to theactuator A₃. A step driving signal generating means A₇ is provided. Whenthe switching means A₅ is off, a signal SG₁ of a predetermined levelwill be generated by the step driving means 7 and will be applied to thedriving circuit means A₆. The objective lens will be brought closer tothe recording medium A₂ (initially set) so as to be in a position belowthe focal distance. The objective lens will be moved away from therecording medium A₂ by steps from the initially set position and will bemoved into the focus pull-in region. First and second comparing means A₈and A₉ have the focus error signal S_(FE) input under the control themeans A₇ to a first input of each comparing means. First and secondvariation point levels V₁ and V₂ correspond to at least two pointpositions in the far and near variable ranges of the objective lens. Thefirst and second variation point levels are input at the other input tothe comparing means A₈, A₉. At least one of these signal levels, forexample, V₂, is set at a signal level corresponding to any positionwithin the focus pull-in region. The outputs SG₂ and SG₃ of thesecomparing means A₈ and A₉ are input to a servo-start controlling meansA₁₀. When both of these respective outputs of the comparing means A₈ andA₉ present a signal showing that the signal S_(FE) from the error signaldetecting means A₄ is equal to or crosses the signal levels V₁ and V₂ ina predetermined order, the switching means A₅ will be turned on and theservo-loop, consisting of the optical system A₁, focus error signaldetecting means A₄, driving circuit means A₆, actuator A₃, and opticalsystem A₁, will be closed.

The present invention is to start the operation of the servo-circuitafter confirming that the objective lens has entered the focus pull-inregion. Therefore, in this embodiment, the variation point of the FEsignal is detected with a microcomputer (which shall be called a CPUhereinafter) 11. In the principal formation diagram shown in FIG. 2, theCPU 11 performs the respective functions of the step driving signalgenerating means A₇, first and second comparing means A₈ and A₉ andservo-start controlling means A₁₀. The CPU is explained in detail in thefollowing.

In FIG. 3, the CPU 11 controls a logical operation part 13, port A 14and a port B 15 through a bus line 12 and connects and an analog-digitalconverter (which shall be called an A/D converter hereinafter) 16 to thebus line 12. A focus error signal S_(FE) output from a differentialamplifier 17 is input into the A/D converter 16. The focus error signalis made by sampling L digital data points at a high speed by a controltiming of the CPU 11. The port B 15 is connected to the D/A converter 19through the bus line 18 and inputs a digital signal into the D/Aconverter 19. The digital signal value is varied synchronously with theabove mentioned control timing as it inputs into the D/A converter 19.

A step driving signal V_(st) for slightly steppedly displacing a laterdescribed objective lens is output by the D/A converter 19. The outputof the port A 14 is to control an on-off switch 20 connected at one endto the output end of the D/A converter 19. The switch 20 is connected atthe other end and to an input and output path of the A/D converter 16and differential amplifier 17 through a phase compensating circuit (PCN)21 and low pass filter (LPF) 22.

The CPU 11 has a memory means for storing program data for carrying outthe program shown by the flow chart in FIG. 4 and for storing variousnumerical value data for detecting reference points in the presentinvention. The stored data causes the objective lens to gradually moveaway from the side near the disc in pulling in the focus servo means.The stored data represents focus error signals S_(FE) shown in FIG. 1.

The level S_(a) at a is determined in response to any position L₃ in therange in which the distance 1 between the objective lens and disk is0<1<L₁. The level S_(a) determined in response to the distance L₃ inFIG. 1 is to confirm that the objective lens is passing the rising part(0<1<L₁) of the focus error signal. The level S_(b) at b is determinedto correspond to any position in the range of L₁ <1<L₂. In FIG. 1, thelevel S_(b) is determined to correspond to the just focus point L_(J).The level S_(b) (=S_(J)) is to confirm that the objective lens is withinthe focus pull-in region. Thus, whenever the focus error signal issampled and the conversion value is output by the A/D converter 16 at apredetermined timing, the CPU 11 will then read out the value to becompared from the memory means and will compare the stored values withthe sampled signals at these variation points a and b. What is importantis not that the focus error signal is merely sampled at fixed intervalsand the servo-means is applied in the logical product in which bothpoints a and b are detected, but the program detecting the point a andthe program detecting the point b are independent from each other. Thatis to say, when the sub-routine of detecting the point a ends, thesub-routine of detecting the point b will be carried out to positivelydetect the point b.

Now, the connecting point of the D/A converter 19 and on-off switch 20is connected to one input end of a driver 23 which is grounded at theother input end. The output of the driver 23 is connected to an actuatorcoil 25 for moving the objective lens 24 which functions as a lightcollecting lens. The lens 24 moves in the directions X and Y indicatedby the arrows. By this formation, the objective lens 24 is moved in theX and Y directions of the disc in response to the output level of thedriver 23.

The reference numeral 26 represents a light source such as asemiconductor laser. A collimator lens 27 is in front of the lightsource 26 so that light beams passing through the lens 27 may passthrough a beam splitter 28 and may be incident on a mirror 29 which isset to lead the reflected light to a disc 30 supported by a turntable.The objective lens 24 is located between the mirror 29 and the disc 30.A light collecting lens 31 is provided on the reflecting surface side ofthe beam splitter 28. The light collecting lens 31 leads the light beamscollected by the light collecting lens 31 onto the flat surface of acolumnar lens 32. The light beam passing through the columnar lens 32are projected onto a quarterly-divided photodetector 33. When theobjective lens 24 and disc 30 are separated from each other by the focaldistance of the objective lens 24 (in the just focus state), asindicated by the one-point chain line, a true circular spot will be madeon the light receiving surface of the photodetector 33. Thus, therespective divided light receiving surfaces will receive equal amountsof light. When the distance between the disc 30 and the objective lens24 deviates from the (just) focus point, as indicated by the two-pointchain lines, an elliptic spot having a longer diameter, corresponding tothe deviating distance, will be made. The long diameter direction of theelliptic spot will coincide with the geometrical direction of the pairof light receiving surfaces of the photodetector. The polarity and levelof the output of the photodetector 33 will vary in response to thedirection and distance deviated from the (just) focus point.

The respective divided light receiving surfaces of the photodetector 33are connected to ground respectively through resistances R₁ to R₄ sothat signal voltages corresponding to the respective received lightamounts may be generated at both ends of the resistances. The voltagesat both ends of the resistances R₁ and R₂ represent signals output by apair of light receiving surfaces on one side of the photodetector. Thevoltages at both ends of the resistances R₃ and R₄ represent signalsoutput by a pair of light receiving surfaces on the other side of thephotodetector. The signals from the resistances R₁ and R₂ arerespectively input into an adder 34 and the signals from the resistancesR₃ and R₄ are respectively input into an adder 35. The output of theserespective adders 34 and 35 are connected to the differential amplifier17.

The present invention is formed as described above. The operation of theinvention will be explained with reference to FIGS. 4 and 1. The FEsignals can be detected by a method wherein the objective lens 24 ismoved away from the disc 30 (from any point close to the focusservo-means pull-in region) or a method wherein the objective lens 24 ismoved closer to the disc 30 from a point far away from the disc (fromany point far away from the focus servo-means pull-in region). In thisembodiment, the former shall be adopted (in the latter, the point a' maybe detected instead of the point a).

First of all, as mentioned above, the point a data of the FE signalinside the pull-in region and the point b data of the FE signal,corresponding to the focus point L_(J) within the pull-in region, aestored in the CPU 11. The output of the port A 14 is set to keep theon-off switch 20 in an off position.

In the initial state of the circuit is thus set, the program, shown bythe flow chart in FIG. 4, will start. The flow chart consists of twosub-routines SR₁ and SR₂ and a branch BL.

The first step N₁ of the routine SR₁ is to clear the data from the portB 15. The port B 15 will input initial data into the D/A converter 19,which will output the data as a step driving signal onto the actuatorcoil 25 through the driver 23. The objective lens 24 will be displacedto the position indicated by the dotted lines in FIG. 3. This displacedposition is a state in which the objective lens 24 has approached thedisc 30 from the point L_(J). At this time, the light beams, havingpassed through the columnar lens 32, will be projected as an expandedbeam (or an enlarged beam) onto the light receiving surface of thephotodetector 33. The FE signal S_(FE) output from the differentialamplifier 17 will be on a higher level than the level at L_(J), that is,the level S_(J) as, for example in FIG. 1. Then, the contents of theport B 15 will be increased by a 1 bit when the process at step N₂ iscarried out. The step driving signal V.sub. ST will be decreased, forexample, by 1 step and will slightly displace the objective lens 24 inthe direction Y. Using FIGS. 5a and 5b, when the port B 15 is increasedby 1 bit, the objective lens 24 will move from a point Lm from the disc30 to the point Ln. The objective 24 will move gradually by the curveshown, for example in FIG. 5b. In FIG. 5b, the distance represented isrepresented on the ordinate and the time is represented on the abscissa.The objective lens 24 will complete the movement to the point Ln att=t_(o). FIG. 5a indicates the FE signal S_(FE) fluctuating with thismovement. The value of S_(FE) is input as a digital value into the ALU13 by the A/D converter 16 a plurality of times (for example, 40 times)with the movement from Lm to Ln (called sampling hereinafter). In FIG.5a, S_(FE1), S_(FE2), . . . S_(FE39), and S_(FE40) are read in asdigital values. The values of S_(FE1) to S_(FE40) are compared in turnat step N₃ with signal S_(FE) at the point a. The step N₃ is todetermine whether S_(FE) is less than the data stored for point a. IfS_(FE) is smaller, the process will move to the next step N₃ but, ifS_(FE) is not smaller, the process will move to the later describedsub-routine SR₂. The step N₄ is to determine the sampling number I, thatis to say, the number of times S_(FE) is smaller than the stored datafor point a. In the embodiment, the sampling number step N₄ is made, forexample, 40 times. Therefore, if the number of times S_(FE) is smalleris summed and the sum of the times is not 40 times (NO), the processwill again return to the comparison step N₃. IF S_(FE) ≧S_(a) isdetermined by the step N₃ before 40 times, the process will move to thesubroutine SR₂. The determination of S_(FE) ≧S_(a) shows that the pointa level in FIG. 1 is detected.

If S_(FE) <S_(a) can not be determined even after 40 times of sampling,the process will return to the step N₂ as the point a is not in therange from Lm to Ln and the point a level will be again detected fromthat position.

The sub-routine SR₂ is a program for detecting a focus error signallevel corresponding to the point b. That is to say, when a negativeresult is obtained from step N₃ of the previous routine SR₁, the CPU 11will move to the step N₅. This step corresponds to the step N₂. When thedata of the port B 15 is further increased by one bit and the stepdriving signal V_(ST) is decreased, the objective lens 24 will befurther stepped closer to the position L_(J) corresponding to the pointb. The step N₆ is made for movement of the lens 24 for the distancecorresponding to 1 bit. This comparison N₆ also corresponds to thecomparison N₃ of the routine SR₁. When S_(FE) >S_(b), the process willproceed to the step N₇ to determine the sampling number a number oftimes. When S_(FE) ≧S_(b), the process will proceed to the branch BL.Step N₇ is a step of the program for determining the number of samplingtimes during the movement of the lens by one step (1 bit). When thesampling number is made I (or 40) times and the point b level is notobtained (YES), the process will return to step N₅. When the samplingnumber is less than 40 and the point b level is not detected, theprocess will return to the comparison step N₆. This routine will berepeated until the point b level is detected. The larger the samplingnumber in the sub-routines SR₁ and SR₂, the higher the precision ofdetected each position. On the other hand, the fewer the samplings, ashorter time will be taken for the focus search. Therefore, a propernumber of samplings is set by matching both the precision desired andthe amount of search time needed.

When S_(FE) ≦S_(b) is determined in comparison step N₆, the process willproceed to the branch BL and the process step N₈ will be carried out.Step N₈ initiates the starting of the servo-means to the port A 14 sothat a signal may be output from the port A 14 to turn on the switch 20.Naturally, this means that the servo-loop consisting of the actuatorcoil 25, objective lens 24, disc surface, objective lens 24,photodetector 33, adders 34, 35, differential amplifier 17, low passfilter 22, phase compensating circuit 21, switch 20, driver 23 andactuator coil 25 will be turned on. At this time, the objective lens 24will be in or near the position of L_(J) in the range of L_(J) to L₂.

Thus, in the present invention, when the objective lens 24 is confirmedto be in the focus pull-in region, the servo-means will be able to beoperated.

The second embodiment of the present invention shall be explained in thefollowing with reference to FIG. 4. The difference from the precedingembodiment is that, in step N₂ of the previous embodiment, the distanceof moving one increment corresponds to one bit. In the secondembodiment, the distance of moving one increment corresponds to aplurality of bits (for example, 3 bits). (A plurality of bits shall becalled one step hereinafter).

In this embodiment, since the variation level per step of the stepdriving signal is higher than in the preceding embodiment, depending onthe selection of the point a S_(FE1) ≧S_(a) may be detected, in thefirst pass through of the subroutine SR₁. The reasons why the samplingintervals may be made so coarse is because the characteristics of thefocus error signal slowly change in the region inside the focal distance(outside the pull-in region W). Thus, even if the sampling intervals areexpanded, this region will not be passed over. In the second embodiment,the lens is moved every 3 bits (1 step) in the routine SR₁ but is movedevery 1 bit in the routine SR₂. However, the invention is not limited tothis. Even in the routine SR₂, a plurality of bits may be moved.

In the respective first and second embodiments, the point b need notcoincide with the just focus point and may be set at any variation pointin the pull-in region. When point b is to be set particularly in therange of L₁ to L_(J) (exactly somewhat below L_(J)), the point a leveland point b level may be the same value, because the present inventionis not to start the servo-means by the logical product merely by thepoint a and b levels being detected. Rather, the routines for detectingthe respective points are independent of each other and the order ofcarrying them out is determined.

The third embodiment can be applied to the critical angle method orknife edge method and shall be explained in the following. The criticalangle method and the knife edge method per se are known methods.

In these methods, the FE signal varies as FIG. 6. Therefore, when thelens is moved in the direction Y from the position of L_(R) near thedisc, the same as in the first and second embodiments, the levels a andb may be detected.

The manner of moving the objective lens 24 in the direction X from anypoint farther away from the disc than the servo-means pull-in region or,for example, from the position of L_(R) ' shall be explained. First ofall, if a substantially average value point c, for example, between S₃and S_(J) is selected as a detecting point outside the pull-in region W,since the characteristics of the FE signal have the minimum point levelS₃, which is lower than S_(J) at a point away from the pull-in region asdescribed above, the S_(J) level will be crossed during the variation tothe maximum point S₂ entering the pull-in region from S₃. The variationpoint presenting the same level as the S_(J) level shall be denoted byd. That is to say, in the third embodiment, the points c, d and b are tobe detected in the order mentioned. The detecting point d may be on anylevel in the range of S₂ to S₃. In order to detect three such points, inthis embodiment, the CPU 11 repeats the same routine as a routine offurther detecting the point d level even after detecting points c and d.

As in the above, in the present invention, the pull-in region is to bepositively detected by detecting the level of any point outside thepull-in region.

The present invention can be appliedd also to an FE signal detectingmethod as a skew beam method.

The fourth embodiment of a focus servo-circuit is provided in whichrecording or reproduction can be made by setting the pickup opticalsystem or disc in a proper state even when the reflection factorfluctuates.

That is to say, in FIG. 3, in the CPU 11, the digital data output to seta focus level through the I/O port B 15 varies in turn and is convertedto an analogue amount by the D/A converter 19. A signal gradually movingthe objective lens 24 away from the disc 30 is applied on the actuatorcoil 25.

The FE signal S_(FE), detected in each position of the objective lens 24moved by a slight amount, is converted to digital data DS_(FE) by an A/Dconverter 16. The signal is operated according to the flow chart shownin FIG. 7 by an arithmetic operation logical unit (ALU) 13, then thenext digital data DS_(FE) is input in turn and the same operation ismade.

That is to say, when a discriminating level setting process begins,objective lens driving data output from the I/O port B 15 will becleared and the respective digital data DS_(FE) MAX and DS_(FE) MIN ofthe maximum value S_(FE) MAX and minimum value S_(FE) MIN of the FEsignal S_(FE) will be set, for example at 0 and 255 in the register ofthe ALU 13 of the CPU 11 or in another register (or an appended memory).Even if the reflection factor of the disc 30 fluctuates, the A/Dconversion level at the point in time when moving the objective lens 24may be within the range of 0 to 255. The objective lens driving (moving)data output from the I/O port B 15 are increased by 1 step (for example,by 1 bit). The digital data DS_(FER) of the FE signal S_(FE) is thenoutput from the photodetector 26 and converted by the A/D converter 16.The output data is compared with the above mentioned maximum value dataDS.sub. FE. A determination is made whether DS_(FE) >DS_(FE) MAX. Whenthe digital data is greater than the maximum (YES), the data DS_(FE)will be replaced with the data DS_(FE) MAX. However, when the digitaldata is less than the maximum value (NO), whether DS_(FE) <DS_(FE) MINwill then be determined. That is to say, when DS_(FE) <DS_(FE) MIN isdetermined to be true (YES), the replacement will be made but, in caseit is determined to be NO, the digital data of the I/O port B 15 will beincreased by 1 step. Thus, the objective lens 24 will be moved by aslight amount from the disc 30. The digital data DS_(FE) of the FEsignal S_(FE) will be determined by the ALU 13 through the A/D converter16 as to whether DS_(FE) >DS_(FE) MAX and DS_(FE) <DS_(FE) MIN in turnas described above and, depending on the outcome of the determination,the replacement with the DS_(FE) MAX or DS_(FE) MIN will be made.

For example, in FIG. 7, in the initial step 0, the objective lens 24 isset at a distance L_(o) from the disc and, the digital data DS_(FE)=DSHD co of the FE signal S_(FE) of the level S_(co) is input. By thedetermination of DS_(FE) (that is, DS_(co)) >0 and DS_(FE) (that is,DS_(co)) <255, in case the respective conditions are established, thereplacement of DS_(FE) MAX =DS_(co) and DS_(FE) MIN =DS_(co) is made.

In the next step 1, the objective lens 24 is moved by a slight amountand is set at a distance L₁. The same determination as described aboveis made by the FE signal in this state, and when the determination isYES, the replacement will be made.

When this operation process is made in turn in the range including atleast the servo-means pull-in region, the maximum value data DS_(FE) MAXwill be set substantially at the maximum value level S_(MAX) (=S₂) inFIG. 6 and the minimum value data DS_(FE) MIN will be substantially atthe digital data corresponding to the minimum value level S_(MIN).

When the maximum value data DS_(FE) MAX and minimum value data DS_(FE)MIN are thus determined, the digital data DS_(b) and DS_(a) of thediscriminating reference levels S_(b) and S_(a) will be determined bythe arithmetic operation in FIG. 7b, 7c, or 7d.

In FIG. 7b, the level S_(b) is set at the average value of the maximumvalue S_(MAX) and minimum value S_(MIN) and the level S_(a) is set atthe average value of the level S_(b) and minimum value S_(MIN).

S_(b) and S_(a) may be set not only as shown in FIG. 7b above but alsoas in FIG. 7c and 7d. In FIG. 7d, "128" means 1/2 of the maximum data ofthe digital conversion data.

The levels S_(b) and S_(a) are determined on the basis of the actual FEsignal S_(FE). Therefore, the levels will be properly set even when thereflection factor varies such as when the disc 30 is replaced. Thelevels will be set on the basis of the FE signal S_(FE) output from thephotodetector even when the photoelectric conversion efficiency or thelike of the optical pickup system or photodetector 26 is different.Therefore the levels will not be substantially influenced by the opticalpickup system or photodetector.

According to the above mentioned fourth embodiment, a discriminatinglevel value can be determined without substantially requiring a memoryor the like for containing data.

When the focus level is set as described above, the focus will besearched according to the flow chart shown in FIG. 4 or the like. Afterpassing the above mentioned level S_(a), at the time when the level isabove the level S_(b), the focus servo-means will be on, a focus loopwill be formed and a just focus state will be quickly set.

In the above description, the objective lens 24 moves in a directionaway from the disc 30 to set the focus level. However, the presentinvention is not limited to this. The objective lens may move in adirection approaching the disc 30.

In this case, it is desirable to have a more positive autofocus than atanother level on the opposite side of level S_(a) outside the focuspull-in region W to use for the determination in focus searching.

The system of setting the respective levels S_(a) and S_(b) is notlimited to the one described above.

This embodiment can be extensively applied not only to optical recordingand reproducing devices such as an optical disc but also to recordingand reproducing devices such as photomagnetic disks.

It is desirable that the discriminating threshold value setting means ofthis embodiment is actually operated prior to the focus search. However,it need not always be operated before the focus search but may beoperated periodically. Also, the controlling level can be held with aback up current source even when the current source is off and can bereplaced with an amended or renewed value when the level is renewed.

The peak value can detect the maximum value through a holding circuitand can detect the minimum value through a peak holding circuit througha reversing circuit.

In the above mentioned first embodiment, the critical angle method hasbeen described. However, the present means can be utilized also forother focus detecting systems such as an astigmatism method.

As described above, according to the fourth embodiment, when the focussearch is actually made, the objective lens is moved. When the FE signalS_(FE) reaches the discriminating level, prior to operating the focusservo-means, the objective lens will be moved in advance. The actualforce error signal waveform level will be examined and thediscriminating level will be determined or amended in response to thewaveform level so that the servo-means may be operated at a properdiscriminating level value without being substantially influenced by thefluctuations of the optical system of the optical pickup, thephotoelectric conversion efficiency of the photodetector and thereflection factor of the disc.

Therefore, the optical pickup can be positively set in a just focusstate and the reliability of the device can be improved.

The device can be applied also to a recording medium which is differentthan the standard reflection factor.

The allowable range of the fluctuation of the optical pickup can be madeso large that mass-production and low cost can be met.

The step width of the step driving signal when the objective lens isgradually moved by the step driving signal, may be set in response tothe level difference S_(MAX) -S_(MIN) between the minimum value S₁ (orS_(MIN)) and maximum value S₂ (or S_(MAX)) on both sides of the focuspull-in region. That is to say, in case the level difference S_(MAX)-X_(MIN) is small, the moving step width may be made small in focussearching and, in the reverse case, the moving step width may be madelarge.

For the step width moving within the pull-in region, the pitch widthnear the middle of the level difference S_(MAX) -S_(MIN), that is, onthe side near the just focus point, may be made narrower than the pitchwidth on both corner sides of the pull-in region.

I claim:
 1. A focus servo-means comprising:an optical recording medium;a light source which generates light; a focus servo-loop meansincludingan optical system for collecting the light generated by thelight source and for projecting said light onto said optical recordingmedium, a focus actuator applies a driving signal to said optical systemto move said opticl system varying distances from said recording medium,a photodetector detects the light reflected from said recording medium,an error signal producing circuit connected to said photodetector, saiderror signal producing circuit for producing focus error signalscorresponding to the light detected by said photodetector, and a drivingcircuit amplifies said focus error signals from said error signalproducing circuit and for outputs the amplified focus error signals asdriving signals for said focus actuator; a switch connected between saiddriving circuit and said error signal producing circuit, said switchopening and closing said focus servo-loop means; a moving signalproducing means for outputting a move signal to the driving circuit tomove said optical system varying distances from said recording medium,said move signal being applied to said driving circuit when said switchis opened; an ordered reference level setting means for setting aplurality of ordered reference levels from the focus error signalcharacteristics, at least one of said ordered reference levels being setwithin a focus pull-in region; a comparing means for successfullycomparing over time said plurality of ordered reference levels with thefocus error signal detected from said error signal producing circuitwhen said move signal is applied to said driving circuit; and aservo-starting means for closing said switch to set a focus servo-statewhen the focus error signal, from said error signal producing circuitwhich has said move signal applied to said driving circuit, is at leastequal to said plurality of ordered reference levels as compared by saidcomparing means, said focus error signal being within said focus pull-inregional when said focus servo-state is set.
 2. A focus servo-meansaccording to claim 1 wherein said moving signal producing means,comparing means and ordered reference level setting means are operatedby a microcomputer.
 3. A focus servo-means according to claim 1 whereinsaid moving signal is made smaller in width with the lapse of timewithin the focus pull-in region than outside said focus pull-in region.4. A focus servo-means according to claim 1 wherein said moving signalis to move said optical system toward the focus pull-in region from aposition closer than the distance between said recording medim and focuspull-in region.
 5. A focus servo-means according to claim 1 wherein saidmoving signal is to move said optical system toward the focus pull-inregion from a position farther than the distance between said medium andfocus pull-in region.
 6. A focus servo-means according to claim 1wherein said moving signal varies steppedly to rise or fall.
 7. A focusservo-means according to claim 6 wherein the focus error signal iscompared a plurality of times together with sampling in the comparingmeans during the movement of the optical system by one step of saidstepped moving signal.
 8. A focus servo-means according to claim 1 whichis provided with an extreme value detecting means which keeps saidswitch open, applies a moving signal, has a focus error signal input anddetermines the maximum value and minimum value of the focus error signaland a reference level setting means setting said plurality of referencelevels.
 9. A focus servo-means according to claim 1 wherein saidreference level within said focus pull-in region is set to be near tothe focus level.
 10. A focus servo-means according to claim 1 whereinthe time variation width of said moving signal is set in response to thelevel difference between said maximum value and minimum value.
 11. Afocus servo-means according to claim 9 wherein the reference levelwithin the focus pull-in region is a mean value of both levels of saidmaximum value and minimum value.
 12. A focus servo-means according toclaim 11 wherein the reference level outside the focus pull-in region isa mean value of the reference level within said focus pull-in region andthe level of said minimum value.
 13. A focus servo-means according toclaim 8 wherein the reference level within the focus pull-in region is amean value of 3 times the minimum value and the maximum value.
 14. Afocus servo-means according to claim 13 wherein the reference leveloutside the focus pull-in region is equal to said reference level withinthe focus pull-in region.
 15. A focus servo-means according to claim 8wherein the reference level outside said focus pull-in region is a meanvalue of half the maximum value level and minimum value level.